N-(phosphonomethyl)glycine is described by Franz in U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a component of aqueous, post-emergent herbicide formulations. As such, they are particularly useful as a highly effective and commercially important broad-spectrum herbicide for killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants.
One of the more widely accepted methods of making N-(phosphonomethyl)glycine products includes the liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl) iminodiacetic acid substrate. As used herein, “N-(phosphonomethyl)iminodiacetic acid substrates” include N-(phosphonomethyl)iminodiacetic acid and salts thereof, wherein the salt-forming cation is, for example, ammonium, alkylammonium, an alkali metal or an alkaline earth metal. Over the years, a wide variety of methods and reactor systems have been disclosed for conducting this oxidation reaction. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. No. 4,624,937; Chou, U.S. Pat. No. 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; Siebenhaar et al., International Publication No. WO 00/01707; Ebner et al., U.S. Pat. No. 6,417,133; Leiber et al., U.S. Pat. No. 6,586,621; and Haupfear et al., International Publication No. WO 01/92272.
The liquid phase oxidation of an N-(phosphonomethyl) iminodiacetic acid substrate typically produces a reaction mixture containing water and various impurities besides the desired N-(phosphonomethyl)glycine product. These impurities may include, for example, various by-products, unreacted starting materials, as well as impurities present in the starting materials. Representative examples of impurities present in N-(phosphonomethyl)glycine product reaction mixtures include unreacted N-(phosphonomethyl) iminodiacetic acid substrate, N-formyl-N-(phosphonomethyl) glycine, phosphoric acid, phosphorous acid, hexamethylenetetraamine, aminomethylphosphonic acid (AMPA), methyl aminomethylphosphonic acid (MAMPA), iminodiacetic acid (IDA), formaldehyde, formic acid, chlorides and the like. The value of the N-(phosphonomethyl)glycine product normally dictates maximal recovery of the product from the reaction mixture and also often provides incentive for recycling at least a portion of the depleted reaction mixture (e.g., to the oxidation reactor system) for further conversion of unreacted substrate and recovery of product.
Commercial considerations also sometimes dictate that the concentration of the N-(phosphonomethyl) glycine product in the commercially sold mixtures be significantly greater than the concentrations in the reaction mixtures that are typically formed in the oxidation reactor system, particularly where the N-(phosphonomethyl) glycine product is being stored or shipped for agricultural applications. For example, when a heterogeneous catalyst is used for the liquid phase oxidation of N-(phosphonomethyl)iminodiacetic acid to make the N-(phosphonomethyl) glycine as described by Haupfear et al. in International Publication No. WO 01/92272, it is typically preferred to maintain a maximum concentration of the N-(phosphonomethyl) glycine product in the reaction mixture of no greater than about 9% by weight in order to keep the product solubilized, although higher concentrations in excess of 9% and even up to about 12% by weight may be suitably utilized at higher reaction mixture temperatures. Sometimes, however, it is desirable for the commercially sold mixtures to have an N-(phosphonomethyl) glycine concentration that is significantly greater. Thus, after the N-(phosphonomethyl)glycine product has been formed and, if necessary, separated from the catalyst, it is typically preferable to concentrate the product and separate the product from the various impurities in the oxidation reaction mixture.
Smith, in U.S. Pat. No. 5,087,740, describes one process for purifying and concentrating an N-(phosphonomethyl) glycine product. Smith discloses passing a reaction mixture containing N-(phosphonomethyl)glycine through a first ion exchange resin column to remove impurities that are more acidic than the N-(phosphonomethyl)glycine, passing the effluent from the first ion exchange resin column through a second ion exchange resin column which adsorbs the N-(phosphonomethyl) glycine, and recovering the N-(phosphonomethyl) glycine by passing a base or strong mineral acid through the second ion exchange resin column.
Haupfear et al., in International Publication No. WO01/92272, describe processes for purifying and concentrating an N-(phosphonomethyl)glycine product prepared by the oxidation of N-(phosphonomethyl)iminodiacetic acid substrates. Haupfear et al. describe generating two crystalline N-(phosphonomethyl)glycine products in two separate crystallizers wherein the crystals have two distinct purities. The lower purity material may then be blended with the higher purity material to produce a single product of acceptable purity.
There remains a need for processes for producing and recovering a crystalline product from a solution comprising a product subject to crystallization and undesired impurities that is capable of producing multiple product mixtures containing the crystalline product, each exhibiting a suitable impurity profile for the intended use. Particularly a need exists for processes for producing and recovering a crystalline N-(phosphonomethyl)glycine product from a reaction solution prepared by the oxidation of an N-(phosphonomethyl) iminodiacetic acid substrate capable of producing both a saleable N-(phosphonomethyl)glycine wet-cake product as well as concentrated liquid or solid salts of N-(phosphonomethyl) glycine of acceptable purity for use in formulation of herbicidal compositions. Such a process would improve overall flexibility to adequately support market demand for various N-(phosphonomethyl)glycine products.